gen_log: pass a struct to a logging callback

gen_log now uses a struct instead of Json object.
- no json deps
  - simplify the build system
- smaller artifacts
  - netd_aidl_interface-V5-cpp.so: 634692 -> 483484

Bug: 161439795
Bug: 172188290
Test: aidl_integration_test
Change-Id: I7de3e10b1221bf83ce46e6636c5aad1457515f21
diff --git a/aidl_to_cpp_common.h b/aidl_to_cpp_common.h
index 2b2db4a..0779cca 100644
--- a/aidl_to_cpp_common.h
+++ b/aidl_to_cpp_common.h
@@ -32,6 +32,8 @@
 // provides _call_toString(expr) which call expr.toString() if possible
 extern char kToStringHelper[];
 
+extern char kTransactionLogStruct[];
+
 // These roughly correspond to the various class names in the C++ hierarchy:
 enum class ClassNames {
   BASE,          // Foo (not a real class, but useful in some circumstances).